Semiconductor devices such as DRAMs have decreased in size and increased in charge storage density dramatically over the last 20 years. As the capacity of DRAM cells has increased and their size has decreased, the design of the cells has become increasingly complex in order to achieve sufficient electrical capacitance to hold the electrical charge representing the stored data.
Traditionally, silicon dioxide has been used as the dielectric in the capacitors of DRAM cells. Silicon dioxide, however, has a relatively low dielectric constant and thus, limited charge storage density. This has resulted in experimentation with the use of materials with higher dielectric constant to increase the electrical capacitance in these very small complex cells.
In recent years ferroelectric materials such as barium strontium titanate (Ba.sub.1-x Sr.sub.x TiO.sub.3)) have been examined for use in dynamic random access memory devices. Ba.sub.1-x Sr.sub.x TiO.sub.3 films are desirable in that they have relatively high dielectric constants (.epsilon..sub.r), ranging from 300 to 800 depending on the value of "x". Ba.sub.1-x Sr.sub.x TiO.sub.3 films are easy to prepare and are structurally stable. Because of their high dielectric constants, Ba.sub.1-x Sr.sub.x TiO.sub.3 films provide almost an order of magnitude higher capacitance density in DRAM cell capacitors than conventional dielectrics such as silicon dioxide. Further, Ba.sub.1-x Sr.sub.x TiO.sub.3 has a low Curie point temperature, ranging between 105.degree. K. to 430.degree. K. depending on the value of "x". This results in a small temperature coefficient of capacitance. Additionally, Ba.sub.1-x Sr.sub.x TiO.sub.3 is not affected by piezoelectric effect because it exhibits a paraelectric phase at room temperature. This opens up the possibility of integrating a Ba.sub.1-x Sr.sub.x TiO.sub.3 capacitor into the existing silicon and gallium arsenide ultra large scale integrated circuit (ULSI) technology to make a commercial dynamic random access memory device.
Several problems still need to be overcome, however, before a commercially viable memory product is available. Foremost among these problems is the degradation of ferroelectric devices due to fatigue, low voltage breakdown and aging. Degradation causes dielectric breakdown of the ferroelectric device and as such results in a decrease in the dielectric constant, thereby decreasing the charge density storage capacity. A common cause of degradation is the interaction between defects in the materials and the ferroelectric-electrode interface/grain boundaries in the ferroelectric capacitor. For example, fatigue degradation, which is one of the prime obstacles to forming high quality ferroelectric films, is caused by defect entrapment in the ferroelectric-electrode interface.
Defect entrapment at the ferroelectric-electrode interface is caused by asymmetric ferroelectric-electrode interfaces and by non-uniform domain distribution in the bulk. Asymmetric electrode-ferroelectric interfaces and/or non-uniform domain distribution in the bulk lead to asymmetric polarization on alternating polarity. This results in an internal field difference which can cause effective one-directional movement of defects such as vacancies and mobile impurity ions. Because the electrode-ferroelectric interface is chemically unstable, it provides sites of lower potential energy relative to the bulk ferroelectric, thereby causing defect entrapment at the interface (see Yoo, et al., "Fatigue Modeling of Lead Zirconate Titanate Thin Films", Jour. Material Sci. and Engineering), resulting in a loss of dielectric constant in the ferroelectric.
To overcome the problems associated with defects it is necessary to control the defect concentration, defect migration to the interface, and defect entrapment at the interface. Defect migration and entrapment can be controlled by reducing the abrupt compositional gradient between the electrode and the ferroelectric. It is also necessary to control the state of the interface itself because lattice mismatch, poor adhesion, and large work function differences between the electrode and the ferroelectric cause the interface to be chemically unstable.
The present invention is intended to overcome one or more of the problems discussed above.